FACTORS AFFECTING INFECTIVITY OF VAIRIMORPHA NECATRIX (MICROSPORIDIA: NOSEMATIDAE) IN TRICHOPLUSIA NI (LEPIDOPTERA: NOCTUIAE)

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Abstract
Ingested dosages of 10.9 and 16.5 spores of Vairimorpha necatrix (Kramer) 7 and 27 weeks old, respectively, killed 50% of third-instar larvae of the cabbage looper, Trichoplusia ni (Hübner). Time to death of 50% of larvae (LT50) decreased from 14 to 3 days with increases in dosage from 5 × 101 to 5 × 106 spores (2-week-old)/Iarva. The LT50 for 5 × 101 spores/larva increased from 14 to 25 days with storage of spores at 4°C for 2 and 93 weeks, respectively. Spores applied to soil lost little activity during cool weather but spores applied in summer were inactivated within 60 days. Desiccation or exposure to sunlight or artificial light inactivated the spores. Rate of development of disease was increased by feeding host larvae a diet containing increased proportions of casein or sucrose. The optimum temperature for disease development in T. ni was 25°C.

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